In recent years, information devices rapidly come into widespread use, and various types of information are held in the information devices accordingly. Hence, it is significant to establish security techniques that restrict, for example, leakage of the information held in the information devices. Various authentication techniques (for example, password authentication, biometric authentication, or card authentication) are suggested. However, in many cases, the existing authentication techniques each execute authentication processing only at a login and involve common problems. For example, if an unauthorized person maliciously uses an information device while an authorized user leaves an installation position of the information device, it is difficult to detect the malicious use.
To solve the problem, continuous authentication techniques that continuously perform authentication processing for users even after the login are suggested in, for example, F. Monrose, and A. D. Rubin, “Keystroke dynamics as biometrics for authentication”, Future Generation Comput. Syst., vol. 16, pp. 351-359, 2000, A. Altinok, and M. Turk, “Temporal integration for continuous multimodal biometrics”, in Proc. Workshop on Multimodal User Authentication, pp. 131-137, 2003, and T. Sim, S. Zhang, R. Janakiraman, and S. Kumar, “Continuous verification using multimodal biometrics”, IEEE Trans. Pattern Anal. Mach. Intell., vol. 29, no. 4, pp. 687-700, April 2007. Representative continuous authentication may be face authentication using a feature vector. The face authentication is provided by registering a feature point group serving as a feature vector group of a face region corresponding to a user in an image, and detecting the user in the image based on the correspondence between the registered feature point group and a feature point group extracted at authentication.
Also, K. Niinuma, U. Park, and A. K. Jain, “Soft biometric traits for continuous user authentication”, IEEE Transactions on Information Forensics and Security (TIFS), Vol. 5, No. 4, pp. 771-780, 2, 2010 discloses color histogram authentication that uses a color histogram of an image in which a user is captured. The color histogram authentication may provide robust continuous authentication even when user's posture is changed, as compared with other system using face authentication and so forth. The color histogram authentication provides continuous authentication, for example, by registering a color histogram of a region (for example, a body region) corresponding to a user in an image, detecting the body region of the user in the image based on a similarity between the registered color histogram and a detected color histogram; and tracking movement of the body region.